Forming biaxially oriented ordered polymer films

ABSTRACT

This invention relates in general to the formation of thick films having a biaxial molecular orientation. Such films are prepared in accordance with the present invention from rod-like extended chain aromatic-heterocyclic ordered polymers. Such films have high tensile strength, modulus, and environmental resistance characteristics. A preferred ordered polymer for use in the present invention is poly (para--phenylenebenzo bisthiazole), (PBT), a compound having the structure: ##STR1## The present invention is also directed to methods and apparatus suitable for producing biaxially oriented films, coatings, and like materials from ordered polymers, preferably PBT.

This is a continuation of co-pending application Ser. No. 780,648 filedon Sept. 26, 1985.

FIELD OF THE INVENTION

This invention relates in general to the formation of thick (i.e.,greater than about 0.10, preferably 0.20 mm) films having a controlledbiaxial molecular orientation. Such films are preferably prepared inaccordance with the present invention from rod-like extended-chainaromatic-heterocyclic ordered polymers. Such films have a controllablecoefficient of thermal expansion (CTE), low dielectric constant, lowmoisture pickup characteristics, low outgassing, high tensile strength,high modulus, and superior environmental resistance characteristics incomparison to uniaxial films of similar composition. The films of thepresent invention exhibit thermal stability, chemical resistance andtoughness, even at low temperatures.

BACKGROUND OF THE INVENTION

Ordered polymers are polymers having an "ordered," orientation in spacei.e., linear, circular, star shaped, or the like, imposed thereon by thenature of the monomer units making up the polymer. Most ordered polymerspossess a linear "order" due to the linear nature of the monomericrepeating units comprising the polymeric chain. Linear ordered polymersare also known as "rod-like" polymers.

For example, U.S. Pat. No. 4,423,202 to Choe, discloses a process forthe production of para-ordered, aromatic heterocyclic polymers having anaverage molecular weight in the range of from about 10,000 to 30,000.

U.S. Pat. No. 4,377,546 to Helminiak, discloses a process for thepreparation of composite films prepared from para-ordered, rod-like,aromatic, heterocyclic polymers embedded in an amorphous heterocyclicsystem.

U.S. Pat. Nos. 4,323,493 and 4,321,357 to Keske et al., disclose meltprepared, ordered, linear, crystalline injection moldable polymerscontaining aliphatic, cycloaliphatic and araliphatic moieties.

U.S. Pat. No. 4,229,566 to Evers et al., describes para-ordered aromaticheterocyclic polymers characterized by the presence of diphenoxybenzene"swivel" sections in the polymer chain.

U.S. Pat. No. 4,207,407 to Helminiak et al., discloses composite filmsprepared from a para-ordered, rod-like aromatic heterocyclic polymeradmixed with a flexible, coil-like amorphous heterocyclic polymer.

U.S. Pat. No. 4,108,835 to Arnold et al., describes para-orderedaromatic heterocyclic polymers containing pendant phenyl groups alongthe polymer chain backbone.

U.S. Pat. No. 4,051,108 to Helminiak et al., discloses a process for thepreparation of films and coatings from para-ordered aromaticheterocyclic polymers.

Ordered polymer solutions in polyphosphoric acids (including PBTcompositions) useful as a dope in the production of polymeric fibers andfilms are described in U.S. Pat. Nos. 4,533,692, 4,533,693 and 4,533,724(to Wolfe et al.).

The disclosures of each of the above described patents are incorporatedherein by reference.

Film processing methods and apparatus have been available for a numberof years. For example, U.S. Pat. No. 4,370,293 to Petersen-Hoj describesa method and apparatus for the manufacture of biaxially oriented plasticfilms, particularly polyester films. The process described for polyestercomprises extruding polyester through an annular die to form a seamlesstube and inflating the tube by means of a pressurized gas. The expandedtube thus formed is drawn out in a longitudinal direction, cooled andflattened. The flattened tube is heated to the orientation temperatureof the film, expanded again, and stretched in its longitudinaldirection. These stretching techniques are said to impart a biaxialorientation to the polymeric backbone of the film.

Similarly, U.S. Pat. No. 4,011,128 to Suzuki describes a method andapparatus for forming a cross-oriented film, wherein a non-oriented filmto be treated is first formed by conventional methods, thencross-oriented by stretching and twisting. In addition thecross-oriented film is flattened so as to continuously form a laminatedcross-oriented film.

U.S. Pat. No. 4,358,330 to Aronovici describes a method and apparatusfor manufacturing films having pairs of adjacent layers whose molecularorientation is in different directions. The method employed is amodification of the conventional "blown film" technique such that themolecular chains forming the layers of film are oriented substantiallyimmediately prior to their solidifying.

U.S. Pat. No. 4,496,413 to Sharps, Jr., describes a process andapparatus for the preparation of a blocked cross-plied polymer filmwhich involves the extrusion of a polymer melt through a tubular rotarydie. The rotation of a single member of the die is said to impart amolecular orientation to the polymer in a transverse direction duringthe extrusion. The film is blocked by expanding the film and thenpressing opposing walls together to produce a composite film having atleast two layers, each having a transverse molecular orientationopposing the other. The composite film is said to have a balancedcross-ply.

The disclosures of each of the above described patents are incorporatedherein by reference.

The degree of molecular orientation achieved during the rotating dieextrusion of thermoplastic polymers is very low, since random coilthermoplastic melts are not oriented to any great extent by shear,unless the melts are anisotropic (such as Xydar). Minimal biaxialorientation of thermoplastics is obtained by blowing tubular films ofthe melt. Even then, the preferential molecular orientation in blownthermoplastic films is in the machine direction.

On the other hand, anisotropic dopes of ordered, rigid-rod polymerscontain isolated bundles of oriented molecules suspended in solvent. Ithas been discovered that counter-rotating tubular extrusion of thesepolymers orients these crystallites in the direction of shear.Stretching of biaxially-oriented tubular films of anisoptropic dope byblowing further increases the degree of orientation in such materials.

SUMMARY OF THE INVENTION

The present invention is directed to the production of films havingheretofore unavailable strength characteristics in more than onedirection. The starting materials useful herein include those lyotropicor thermotropic polymeric materials in which strain produces a materialorientation in the microscale structure and which are relatively weak ifthis orientation is in only one direction, i.e., uniaxial. The presentinvention is particularly applicable to dopes and like materials madefrom ordered polymers, or other rigid rod-like molecules.

The method of the present invention comprises first producing a certainmicroscale structural orientation within a polymer dope by a sequence ofstraining methods, followed by solidifying this ordered structure by asequence of thermal and/or chemical conditioning operations.

The present invention is especially directed to biaxially orientedfilms, coatings and like materials formed from ordered polymers. Apreferred ordered polymer for use in the present invention is poly(para-phenylenebenzo bisthiazole), (PBT), a compound having thestructure: ##STR2##

Biaxially oriented polymeric films of PBT are especially preferredembodiments of the present invention. These films possess uniqueproperties including:

(a) high tensile strength (most preferably, greater than 100,000 psiultimate tensile stress in one direction and not less than 40,000 psiultimate tensile stress in any direction);

(b) high modulus (most preferably, greater than 5×10⁶ psi tensilemodulus in one direction and not less than 8×10⁵ psi tensile modulus inany direction);

(c) controllable coefficient of thermal expansion (CTE) either negative,positive or zero in any particular direction in the plane of the film;

(d) low dielectric constant (most preferably, less than 3.0);

(e) low outgassing (most preferably, less than 0.1% weight loss in avacuum at 125° C. for 24 hours;

(f) low moisture pickup (most preferably, less than 0.5% weight gain inwater at 100° C. for 24 hours.

The present invention is also directed to methods and apparatus suitablefor producing biaxially oriented films, coatings, and like materialsfrom ordered polymers, preferably PBT.

The preferred films, methods and apparatus of the present invention aredescribed in greater detail in the accompanying drawings and in thedetailed description of the invention which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting the process of the present inventionfor the formation of biaxially oriented films from ordered polymers.

FIG. 2 is a schematic representation of a single screw extruderapparatus for the degassing and preconditioning of PBT dope;

FIG. 3 is a schematic representation of a counter rotating tube dieapparatus for producing a biaxially oriented film from an orderedpolymer.

FIG. 4 is a schematic representation of one preferred drying/heattreating apparatus used for producing a biaxially oriented film from anordered polymer.

FIG. 5 is a schematic representation of an apparatus incorporating thedie of FIG. 3 constructed in accord with the present invention.

FIG. 6 is a schematic representation of a counter rotating plateapparatus for producing a biaxially oriented film from an orderedpolymer;

FIG. 7 is a schematic representation of a roller die apparatus forproducing a biaxially oriented film from an ordered polymer.

FIG. 8 is a schematic representation of the processing apparatuspreferably employed in the present invention.

FIG. 9 illustrates various orientations of polymer films. FIG. 9Arepresents uniaxial orientation, i.e., that imposed on polymers bytypical slit-die extrusion or fiber spinning. FIG. 9B represents therandom disorder of ordered polymer films that are coagulated withoutpre-orientation. FIG. 9C illustrates the biaxial order imposed on thepolymer of FIG. 9B by treatment in accord with the present invention.

FIG. 10 illustrates an end-attachment for the tube-die of FIG. 8 toreduce the die gap thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to the production of film havingcontrolled anisotropic properties from ordered polymers.

When ordered polymers are subjected to a shear field they become highlyaligned in the direction of the applied field. By imparting to suchpolymers a preferred orientation one obtains material with a hightensile value which is the basis for producing fibers of high strength.

Similar orientation imparted in the machine direction during theproduction of ordered polymer films results in films having a very hightensile strength in the machine oriented direction but very poorphysical properties in the transverse direction. In some cases highlyoriented polymer films will lose their film integrity by simply comingapart along the machine direction orientation.

The present invention is thus directed to the production of orderedpolymer films that have highly controlled orientation resulting in filmsthat have property balances that are much more useful from a practicalstandpoint. Films can be produced having high tensile values in themachine direction and substantial strength in the transverse direction.These films maintain their film integrity and as a result are useful inmany applications requiring good film properties.

The process of the present invention affords films that have strengthcharacteristics making them suitable for the production of laminate filmcomposites and like structures.

The essential strength characteristics of these films are the result ofa two stage orientation process followed by post treatment to optimizethe film property balance. In preferred embodiments, the biaxialmolecular orientation is achieved by utilizing a homogenized dopeconsisting of PBT in polyphosphoric acid. The term "polyphosphoric acid"as used herein, means any of the members of the continuous series ofamorphous condensed phosphoric acid/water mixtures, generally given bythe formula:

    H.sub.n+2 P.sub.n O.sub.3n+1

wherein the value of n depends upon the molar ratio of water tophosphorous pentoxide present. Such compositions are described in U.S.Pat. Nos. 4,533,692, 4,533,724, and 4,533,693 (to Wolfe et al.).

Referring to FIG. 1, there is illustrated a block diagram of theprincipal steps of the method of the present invention for the formationof biaxially oriented films from the preferred ordered polymer, PBT.

As illustrated at 10 the first step comprises a conditioning of thepolymer which preferably is about a 10 to 30 weight percent solution inpoly(phosphoric acid), or PPA. PPA is the preferred solvent, althoughmethanesulfonic acid (MSA) or chlorosulfonic acid (CSA) may also beused. The degassing step is employed to prevent interference ofentrapped gas within the polymer solution with the molecular orientationof the film.

The second step (12) comprises the orientation step. This may beaccomplished by the use of any of the extrusion means which induce shearflow, stretching, and the like. Preferred extrusion means of the presentinvention include counter rotating tube dies, plates, or roller dies. Ithas been discovered that such extrusion means, preferably combined withsubsequent stretching of the extrudate, may be employed to impartvarying degrees of biaxial orientation to ordered polymers.

A third step (14) comprises coagulation of the polymeric solution.

The fourth step (16) is a densification step wherein the PPA is removed.

The penultimate step (18) is generally a drying and heat treatment step.

Finally at step (20) the product film is packaged.

Each of these general steps will be elucidated further in thedescription of the preferred apparatus for conducting the abovedescribed processing conditions.

In FIG. 2 there is illustrated one preferred embodiment of an extruderapparatus for the degassing of PBT dope. After homogenization (asdescribed in the Wolfe et al. patents) the dope is fed by means of aheated pressure pot (22) to the inlet of a slow heated extruder (24)which in turn feeds a positive displacement pump (26).

The positive displacement pump (26) of FIG. 2 feeds a film die (28) asillustrated in FIG. 3. The film die (28) has two counter rotatingbarrels, 30 and 32 respectively, whose purpose is to create a shearfield through the cross section of the extruded dope composition. Thisshear field is at right angles to the axial shear field produced byforcing the dope axially through the annulus of the die. Counterrotating die members are necessary to prevent a screw-like rotation ofthe orientation and twist-off of the extrudate which occurs if only onemember of the die is rotated. This combination of shear fields isnecessary prior to the blowing operation in order to permit blowing ofthe tube without fracturing the extrudate, and hence, to producematerial with integral biaxial film properties.

Upon exit from counter rotating die (28) the film is treated to ablowing operation. Here, the film is expanded under internal pressure,further orienting the molecules throughout the film cross section.Control of the die RPM, extrusion rate, film windup rate, and degree ofexpansion results in a precisely aligned, blown PBT dope compositionfilm. The top and bottom surfaces of the film are aligned atapproximately equal but opposite angles to the machine direction.

As described above, the processing variables of die speed (RPM),extrusion rate, and degree of film extension and expansion during theblowing operation, all can be varied to achieve any desired degree ofbiaxial molecular orientation.

Control of the degree of molecular orientation results in attractivefilm properties. Blown dope compositions that have not been subjected tocontrolled shear fields prior to expansion do not have physical propertybalances anywhere approaching those of the films of the presentinvention. Furthermore, films extruded by the counter rotated die butnot with the blowing process do not have good property balances. It isthe combination of shear field extrusion followed by internal expansionand extension that yields films with a useful property balance.

The extruded, sheared and blown film is quenched, both on the internaland external surfaces, by an aqueous coagulation bath or othercontrolled aqueous coagulant composition. This quenching operationserves to "gel" the polymer dope composition, producing a strong, tough,solution-filled film. By controlling the composition of the coagulationbath many other materials can be incorporated into the filmmicrostructure.

In addition to causing the film microstructure to gel and become strong,the aqueous solution serves to hydrolize the polyphosphoric acid tophosphoric acid, facilitating its removal from the film. Thesolution-filled film is then washed free of phosphoric acid before it issubjected to controlled drying conditions.

As illustrated in FIG. 4, the film is preferably dried under controlledinternal pressure, also known as a restrained drying process. This isaccomplished by drying the film under a regulated air or nitrogenpressure of from about 5 to 10 psi as illustrated. The pressurized filmtube in the example may have about 1.5 to 3 inches diameter and a lengthof from about 5 to 12 inches. Drying under such conditions results in ahighly oriented film of high strength characteristics.

FIG. 5 illustrates schematically the above described processing steps.As illustrated, the conditioning and degassing step is conducted by theapparatus (34), which sends the homogenized dope to the extruder means(36) whereupon shear is imparted to the dope. The dope is then blownusing conventional film blowing equipment (38) and the blown tube enterscoagulation zone (40). The coagulation zone (40) comprises a water tank(41) and may include additives useful in imparting specializedcharacteristics to the film. The coagulation zone acts to stabilize themolecular orientation imparted to the film by the extrusion and blowingprocesses. The water and/or additives in the bath spread into themicrostructure of the film. Following the coagulation zone, there isshown an exchange bath (42). Here the acid solvent used to prepare thepolymer dope (PPA, MSA, CSA, etc.) is removed by repeated waterwashings. Following removal of the acid solvent from the film, the filmcan be exposed to other solutions that may include additives useful inimparting special characteristics to the film. Afterward, the film isdried under appropriate stress conditions in a drying oven (44). Afterdrying, the film is packaged using conventional means (46).

When tube-blowing is employed, if the tube is not slit after coagulationbut is merely collapsed flat for water-solution and drying treatments,it can then be re-blown and stretched biaxially in a tower- ortunnel-oven. The tube is slit into tape and roll-packaged justdownstream of a central plug mandrel and guide rolls. Tube-blowing gasis advantageously introduced through the mandrel.

FIG. 6 illustrates another means of imparting shear stress to thepolymer dope that is useful in the method of the present invention. Asillustrated, the stress means (48) comprises counterrotating pressureplates (47 and 49). Polymer dope, such as PBT is inserted between theplates, pressure is applied and the plates are rotated in oppositedirections.

Another means for imparting shear stress to a polymer dope in accordwith the method of the present invention is the apparatus illustrated inFIG. 7. As illustrated, a laterally spreading die (50) having open topand bottom is contoured to fit in the convergence of two pinch rolls (52and 54). The extrudate enters the die as a high and narrow flow, thenundergoes progressive lateral and axial direct strains to emerge as athin and wide strip. This strip then undergoes some further axialextension to become a film on one of the rolls, depending on the balancebetween roll surface velocity and supply pressure induced flow. Processvariables include the proportions and internal shape of the die, thesupply pressure, and the film tension.

The apparatus illustrated in FIG. 8 represents a counter-rotating tubedie which comprises a rotatable cylindrical inner shaft (56) having asmooth surface encased in an independently rotatable cylinder (58)having a plurality of passageways therein (59). A space (60) is providedbetween the shaft and the cylinder to allow for the introduction ofpolymer and to allow independent movement of shaft (56) and cylinder(58). Cylinder (58) and shaft (56) are rotated in opposite directions.Ordered polymer is fed through passageways (59) to the space (60)between shaft (56) and cylinder (58). The polymeric mass strikes shaft(56) and is subjected to orientation forces by the opposing movement ofcylinder (58) and shaft (56). Drive gears (62) and (64) are shown,attached to outer cylinder (58) and inner shaft (56) respectively. Alsoillustrated is housing (66) which surrounds the tube die and serves tocontrol the temperature of the extrusion system. Water inlet (68) andoutlet (70) are provided to maintain the composition of the coagulationzone and to exert pressure on the interior of the blown film tube.Nitrogen inlet (72) serves to maintain an inert atmosphere within thedie and also provides the means for blowing (i.e., expanding) the filminto a tube upon its exiting the die. Drive gears (62) and (64) areoperated by independent variable speed drive means such as electricmotors (not shown).

Ordered polymer is pumped through passageway (74) in housing (66)whereupon it impinges upon the surface of rotating cylinder (58). Thepolymer flows through the plurality of passages (59) in cylinder (58)into the space (60) between cylinder (58) and rotating shaft (56). Sincethe top of the die is sealed, the polymer flows to the outlet at thebottom (76). As the polymer flows toward the outlet (76) counteractingshear forces imparted by revolving cylinder (58) and revolving shaft(56) impart a degree of biaxial orientation to the film's molecularstructure.

Previous attempts to rotate only one cylinder of the tube die whilemaintaining the other in a stationary condition caused uncontrolledtwisting and tearing of the dope extruding from the die.

Transverse shear, longitudinal flow shear, axial stretch, and radialexpansion forces all interact in the dies illustrated in FIGS. 3, 6, 7and 8 to impart a partial biaxial orientation to the ordered polymer fedtherethrough. Variation of the speed of the movement of the shaft andcylinder of the illustrated die, as well as flow rate, temperature, etc.effect the degree of orientation imparted to the ordered polymerfeedstock. Additional orientation is imparted to the extruded film byvirtue of the blowing processes, both following the extrusion and as apart of the heat treatment.

FIG. 9 illustrates the various orientations imparted to polymers bystress conditions. Typically, polymers subjected to shear stress assumea uniaxial orientation as illustrated in FIG. 9A. Ordered polymers insolution have the scattered or random nematic orientation illustrated inFIG. 9B. FIG. 9C illustrates the twisted nematic (or cholesteric)orientation imparted to ordered polymers by processing under the methodof the present invention.

In the preparation of twisted nematic orientation with PBT by solutionprocessing, molecules in adjacent planes with twisted orientation arenot able to pack closely on solvent removal. Thus, each "layer" willhave to densify by diffusion transverse to the rod axis, an unlikelyprocess on the microscopic scale of the sheet. Consequently, if twistednematic orientation is smooth and gradual through the film thickness,the densification can occur with the least amount of strain ordisruption between adjacent layers.

Biaxial shearing as well as biaxial direct stresses and strains can beimposed and controlled in this system. A useful combination of strainpatterns is achieved by the apparatus of FIG. 8 where first a twistednematic (cholesteric) orientation is promoted in the dies and then auniform biaxial strain is promoted in the blow/stretch. The formerprovides enough bi-directional strength for the latter, as well asnear-order of layers, conducive to densification in the normal(thickness) direction. The biaxial strain can be symmetric orasymmetric. If this system is operated with low strain in the dies, thenbiaxial blow/stretch will promote biaxial nematic orientation ratherthan twisted nematic.

Of course, the system of the present invention could be used to produceuniaxial nematic tube or film as well.

A common characteristic of laminates of the preferred biaxial filmmaterials is that they can be weak in the transverse direction (i.e.,perpendicular to the plane of the laminated film). It is thereforedesirable to increase the so called trans-laminar strength of biaxialfilms by using additional processing steps in the manufacture of thefilms. These additional steps can be during the preparation of the dopeor in the washing or solution processing of the coagulated film.Trans-laminar strength of the film can be increased either by increasingthe cohesivity between the ordered, rigid-rod polymer structure, and/orby enclosing the ordered structure in a binding, surrounding network ofthe added material. This added material typical does not interfere withthe rest of the processing steps, because the added material is notrendered strong and cohesive except by a subsequent processing step,e.g., heat treating or chemical conversion.

An important aspect of the methods envisioned for increasingtrans-laminar film strength is that the added material is notnecessarily intended to be a major fraction of the final structuralmaterial or film; the added material can be a very minor constituent ofthe final structure and still provide substantial trans-laminarcohesivity or strength. In fact, since the rigid-rod ordered polymericstructure is relatively very competent, the added material mostpreferably is a very minor component, such that the final overallmaterial has the highest specific strength and stiffness properties,i.e., highest strength and stiffness per weight and volume.

One method of increasing the trans-laminar strength of biaxial PBT filmis to blend a finely divided powder of compatible material with the PBTdope during the dope-preparation step of the total process. A preferredmaterial is polyphenylene sulfide (PPS), at about 10 percent by volume(or more) of the final dope. PPS is a strong, highly resistant,thermotropic polymer. This powder remains in the dope and the preparedfilm through all of the processing steps up to the final drying stage.During drying and heat treating, the film is heated to a temperaturethat melts the PPS, causing it to flow around and between the PBTrod-like microscale structure. Subsequent pressing or rolling andcooling produces a structure that is strong in all directions of stress.

Another method of increasing translaminar strength is to diffuse aprecursor of a strong binder material into the PBT film during thewashing stage of the process. This precursor can be an organometallicprecursor of an inorganic glass, such as tetramethoxysilane; or anorganically-modified glass precursor that has reactive organic groupsincorporated therein, such as expoxides; or a precursor of athermotropic plastic, such as caprolactam as a precursor for nylon, orpolyamic acid as a precursor for polyimide. After the precursor hasdiffused into the washed but still swollen PBT film, e.g., varioussequential solvent exchanges, the film is dried and heat treated,causing a transformation of the added material to its final form as astrong trans-laminar binder material. As a final binder material,glasses and polyimides are preferred over nylons, because the formermaterials more nearly complement the high temperature and strengthproperties of the PBT film structure.

The processing equipment of the present invention is straightforward indesign and fabrication, with the exception of the counter-rotating dieassembly. The storage tank must be heated, is preferably made ofstainless steel (e.g., type 3l6L suitable for PPA processes), and ispressurized with dry/inert gas (e.g., N₂) in order to prevent bothcoagulation of PBT and/or starvation of the pump. The pump is typicallya precision-gear type (e.g., Zenith). Other types of pump, such aspiston-ram, extruder, or traveling-cavity (Moyno), are possible.

While other counter-rotating tube-dies exist, the design of the die ofthe present invention is specialized in that a wide range of parameterscan be explored by using different speeds and die-inserts. Sealingbetween the hot block and die cylinders is affected by spring loadedface-bushing (Teflon® or graphite), and alignment is maintained byremote collar bearings. Because the extrudate undergoes so muchdensification to final thickness, the die annulus is usually large,moderating die pressure required. The central gas for film blowing (N₂)is provided through a remote, cooler, standard rotating coupling.

Function and operation of the extrusion-blowing system are thusstraightforward:

Counter-rotation of the dies generates transverse shear without any nettwist or torque on the extruded tube.

The pump generates the axial flow and, in combination with the annulargap, determines the axial shear (flow profile).

Draw-down of the tube at a linear rate greater than die-discharge causesan axial strain in the hot, uncoagulated extrudate.

Blowing of the film tube causes circumferential stress and strain in theextrudate.

Immersion in a water bath after blow/stretch causes coagulation and,below the central water level, a balance of pressure and nulling ofpressure differential, unless the tube is pinched closed at the bottom.

Key processing parameters for successful extrusion of biaxial film fromPBT/PPA dope with a tube die substantially as depicted in FIG. 8 arelisted in Table I. This tube die has adaptors at the exit of the die toallow for two different annular diameters and gap distances. Referringto FIG. 8, the shear zone length is the distance between the inletpassageways (59) and the exit of the tube die (76). Shear rates arecalculated as the linear velocity difference between the revolvingcylinders divided by the gap distance. Blow ratio is defined as thefinal diameter of the coagulated PBT/PPA tube divided by the initialdiameter of the PBT/PPA tube at the exit of the die. The draw ratio isdefined as the linear bulk velocity of the PBT/PPA extrudate, at theexit of the die, divided by the wind-up roller linear velocity,referring to (Part 46) of FIG. 5. The linear bulk velocity is defined asthe volumetric output of the extruder dived by the cross-sectional areaof the annular gap of the die. For typical PBT/PPA dope, the extruderzone temperature was 120° C. and the die zone temperature was 80° C.;i.e., the PBT/PPA dope was cooler in the die than the extruder.

                  TABLE I                                                         ______________________________________                                        Tube Die Specifications:                                                      Annular Gap           0.040", 0.080"                                          Annulus Diameter      0.80", 1.5"                                             Shear Zone Axial Length                                                                             4 inches                                                Processing Conditions:                                                        Shear rate            1s.sup.-1 to                                                                  greater than                                                                  3s.sup.-1                                               Blow ratio            1:1 to 3:1                                              Draw ratio            8:1 to 20:1                                             Extruder Temperature  120° C.                                          Die Temperature       80° C.                                           ______________________________________                                    

Applications of the high-strength, high-modulus, thermally-stable,chemically resistant, microporous PBT polymer films of the presentinvention include the following: (1) multi-layered, structuralcomposites molded to complex shapes, (2) rigid, glass-containingcomposites, (3) filters of controlled porosity for use in harshenvironments; (4) gas separation membranes; (5) water-purificationmembranes; (6) electronic circuit board structures; (7) lightweightspace structures; (8) multi-layered, electrically conducting structuralcomposites; (9) ionizing radiation-resistant composites; (10) low radarprofile structures; (11) zero coefficient of expansion structuralcomposites; (12) porous substrates for controlled release of volatilematerials in harsh environments; (13) leaf springs, helical springs and(14) capacitors.

The method of the present invention will be further illustrated withreference to the following examples which are intended to aid in theunderstanding of the present invention, but which are not to beconstrued as a limitation thereof. All percentages reported herein,unless otherwise specified, are percent by weight. All temperatures areexpressed in degrees Celsius and are uncorrected.

EXAMPLE 1

The coagulation and take-up system substantially as described in FIG. 5was used, and blown tube films were extruded under the followingconditions:

    ______________________________________                                        extrusion die:   3.81 cm diam. × 1.02 mm gap                            extrusion rate:  3 cc/min                                                     air gap:         11.7 cm                                                      coagulation zone:                                                                              18.8 cm                                                      take-up speed*:  24.6. cm/min.                                                counter-rotating shear rate:                                                                   4 sec.sup.-1                                                 blow-up ratio:   1.5:1                                                        draw ratio:      10:1                                                         ______________________________________                                         *speed with empty package roll                                           

The PBT/PPA dope of this example had an intrinsic viscosity (IV) of 19as measured by the method described in the Wolfe et al. patents (supra).

An attachment was made for the tube die to reduce the extruded tubediameter to 2 cm (see FIG. 10). This allows greater blow-up ratios usingthe same take-up system, which is limited to a 7.6 cm maximum bubblediameter. The die gap was 1.02 mm and the counter rotating shear ratewas about 4.5 sec⁻¹.

The die was also operated at the full 2.04 mm gap, without any endfixtures, to determine whether the variation in extruded wall thicknesswas due to the inner and outer mandrels of the die, or to theattachments. These films, which are twice as thick (approximately 0.003in, 0.076 mm), are fairly uniform in thickness once the system reachessteady-state, and they do not exhibit any spiral pattern.

Uniform operation can be restored by a combination of reducing internalpressure, increasing longitudinal draw, decreasing internal waterheight, and spraying thin sections with water to "freeze" that sectionof the bubble. Other blown tube processes (high molecular weightpolyethylene, for example) encounter similar bubble stability and filmthickness problems. Internal mandrels (within the bubble) can be used todirect cold air at the blown film to chill it (analogous tocoagulation). Driven pinch rolls could also be used to provide morecontrollable draw.

Converging plates were used on one run to collapse the bubble and reducefolding and creasing of the tube. The plates were made from clearacrylic sheets and were attached about 2 cm above the pinch so that thetops of the plates were above the water level and the PBT film would becoagulated before touching the inclined plate. In operation, thecoagulated PBT tube tended to stick then slip on the plates, causingsome vibration in the take-up system. This was the result of unexpectedfriction between the tube and the plates, and could be remedied by usingTeflon plates, or going to a roller or belt converging system.Otherwise, the converging plates worked well to maintain bubble diameterand alignment, and resulted in smooth surface films.

Washing and Drying Films

All film samples were collected on a wide spool under water and werekept under water without air contact and interleaving corsely wovenmaterial was used to allow water circulation. Samples were washed for atleast 48 hours before drying. The samples measured 0.8% phosphorousafter 24 wash, and 4% on samples with only 5 minutes wash.

Several drying methods were attempted including:

Clamping wet films in 7.6 cm square frames

applying internal pressure of 5 to 9 psi to the wet tube

using rods inside the tube with variable

spring-load between the rods.

The clamped frame method works well, is simple and is convenient to holdsamples for subsequent heat treatement trials. The internal gas pressuremethods requires a pressure regulator, crimping seal at two ends of thetube and a pressure relief valve to allow passage of gas and water fromthe inside of the tube. Internal pressure causes thin film sections tobe more highly stressed than thick ones, but the stress is morepredictable than in the case of the clamping frame. Stress wascalculated during drying trials at 3000 to 5000 psi in the hoopdirection, and half that in the longitudinal direction.

EXAMPLE 2

The PBT/PPA dope of this example was obtained from DuPont, andidentified as follows:

SRI code 5103-28

50 KG (110 lb) of PBT/PPA

13.7% PBT

Intrinsic viscosity (IV)=40, as measured by SRI

DuPont has measured 35 to 40, indicating variability in the dope. Theviscosity is stable with temperature, they report).

82.7% p₂ O₅

These material are much more viscous than the 19 IV dope used in Example1.

The dope preparation system was assembled as in Example 1, with vacuumdegassing, a 50-hole (0.36 mm diameter) spinneret, and a sintered metalfilter (80 micron) spin pack. The major difference between this andExample 1 (using 19 IV polymer) was than the feed pot piston pressureand temperature were increased. The 40 IV dope required 100 psi insteadof 20 psi, and 240° F. instead of 200° F. The flow from the feed pot tothe extruder was slower because there was almost no shear on the dopeand the viscosity remained quite high. Once in the barrel of theextruder, the screw provided shear, and the 40 IV material extrudedeasily at about 230° F. and 1000 psi barrel pressure--very similar toconditions with the 19 IV polymer. The extruder was operated at 3cm/min; faster rates may require more temperature and pressure at thefeed pot to avoid starving the extruder.

Degassing proceeds in much the same way as the 19 IV dope. The filamentsfalling into the vacuum showed "graininess" indicating entrained gaseswere being removed. About 2.5 L (5 kg or 11 lb) of PBT/PPA dope wasprepared by one pass through the system.

As described in Example 1, the tube die was modified to reduce bearingclearances, improve alignment, and reduce corrosion problems.

About 50 ft of high quality film was produced from this example.Relatively high draw ratios were used, in the range of 13:1 to 21:1because bubble stability improved markedly when take-up was increased.Blow-up ratios were maintained at about 2:1. All successful film trialswere made with the 0.040 in. gap. These conditions resulted inrelatively thin film, 0.1 to 0.6 mil (2.5 to 15.2 micron) in thickness.These thin films could be produced consistently at steady-state becauseof the "toughness" of the 40 IV polymer dope and the improved precisionof the die.

The die rotation rate was varied from 0.5 to 2 rpm with a dietemperature of 190° F.

The most successful extrusion conditions are summarized below:

    ______________________________________                                        throughput:       3 cc/min                                                    barrell pressure: 1000 psi                                                    die pressure:     50 to 75 psi (estimated)                                    barrel temperature:                                                                             230° F.                                              die temperature:  190° F.                                              draw ratio:       13:1 to 21:1                                                blow-up ration:   2:1                                                         die rotation:     0.5 rpm                                                     ______________________________________                                    

An ice-water coagulation bath was used to achieve better filmproperties. The low temperature bath provided a slower coagulation whichwas less disruptive to the oriented PBT polymer network.

Film Property Measurements

Tensile tests showed improved strength and modulus for biaxiallyoriented films. Heat treatment was conducted at 400° C. for 2 hours.Higher temperature heat treatment will be evaluated at temperatures upto 650° C. for brief periods, typically 30 to 60 seconds.

EXAMPLE 3 Roller Die Extrusions

In this example about 3.0 liters of 13.7% solids PBT/PPA dope (40 I.V.)was extruded using a roller die substantially as illustrated in FIG. 7.The processing conditions were those listed in Table II. Approximately1.0 liter of dope was extruded in each of the three trial extrusions.

                  TABLE II                                                        ______________________________________                                        Processing conditions for roller-die extrusions                               of 40 I.V. PBT/PPA dope                                                       Run  Throughput cc/min                                                                            Draw    Die Temperature                                                                           °F.                            ______________________________________                                        1, 2 4.8-8.0        --      190° 230°                           3    12.8           4.6:1   "           230°                                15.8           3.8:1   "           230°                           ______________________________________                                    

The first two runs gave useful operating information on the roller diesystem, but did not produce high quality film. The roller die requiredhigher throughputs than the tube die used in the previous examples, andat throughputs around 10 cc/min, the feed vessel could not feed theextruder screw sufficiently. Therefore the feedpot system was modifiedfrom the air-drived piston of the previous Examples to ahydraulic-ram-driven one.

The uncoagulted PBT/PPA extrudate could be released from the rollers ofthe die, with application of a mold release agent "ReleaseaGen H-1501",formerly produced by General Mills.

In run #3, about 15-20 feet of 2 in. wide×0.075 in. thick (washed state)sheet-like PBT was extruded. This thick film had a regular V-shaped,ridged pattern. By visual inspection, the film had a microfibrillarstructure in the machine direction. However it was more difficult tosplit parallel to the machine direction in comparison to a uniaxialfilm. The film thus has the desired improvement in transverse strength.

The present invention has been described in detail, including thepreferred embodiments thereof. However, it will be appreciated thatthose skilled in the art, upon consideration of the present disclosure,may make modifications and/or improvements on this invention and stillbe within the scope and spirit of this invention as set forth in thefollowing claims.

What is claimed is:
 1. A method of preparing biaxially ordered polymerfilms comprising the sequential steps of:(a) treating a dope containingan ordered polymer with simultaneous biaxial shearing forces, therebyproducing a film having at least two microscale structural orientations;(b) treating the film obtained in step (a) with cross-directionalstrains comprising transverse and longitudinal extensions, therebyimparting additional microscale structural orientation to the film; and(c) solidifying the film obtained to retain the microscale structuralorientation imparted thereto.
 2. The method of claim 8, wherein saidshear forces of step (a) are imparted by counter-rotating die membersand longitudinal flow between said die members.
 3. The method of claim1, wherein said physical microscale solidification means comprisestreatment of the stressed dope in an aqueous coagulation bath.
 4. Themethod of claim 1, wherein the thermal microscale solidification meanscomprises drying the tubular film under a positive pressure.
 5. Themethod of claim 1, further comprising tube blowing of the stressedordered polymer dope, said tube blowing imparting an additionalmicrostructure strain to said dope.
 6. The method of claim 1, whereinsaid chemical microscale solidification means comprises treating thedope with a chemical additive.
 7. The method of claim 6, wherein saidchemical additive is polyphenylene sulfide.
 8. The method of claim 6,wherein said chemical additive is caprolactam.
 9. The method of claim 6,wherein said chemical additive is polyamic acid.
 10. The method ofpreparing a biaxially ordered polymer film comprising the sequentialsteps of:(a) pretreating a solution of ordered polymer dope by heatingto a temperature within its orientation range and degassing said heatedpolymer dope; (b) extruding a film from said degassed polymer such thatsimultaneous biaxial shearing forces act upon the polymer, imparting abiaxial microscale structural orientation to the film; (c) subjectingthe film obtained in step (b) to cross-sectional strains comprisingtransverse and longitudinal extensions thereby further orientating saidfilm; and (d) solidifying the film obtained to retain the microscalestructural orientation imparted thereto.
 11. The method of claim 1,wherein step (c) comprises the sequential steps of:(i) coagulating saidoriented film; (ii) washing said film; and (iii) drying and densifyingsaid film.
 12. The method of preparing a biaxially oriented orderedpolymer film comprising the sequential steps of:(a) extruding an orderedpolymer dope such that biaxial shearing forces simultaneously act uponthe dope, creating a film and imparting a first biaxial orientationthereto; and (b) subsequently stretching said film, thereby imparting asecond diaxial orientation thereto; (c) solidifying the film obtained toretain the biaxial orientation imparted thereto.
 13. A method ofpreparing an ordered polymer film comprising: conditioning a dope of anordered polymeric material in a solvent to remove entrapped gastherefrom; extruding the conditioned material through a counter rotatingdie into a film having at least two microscale structural orientations;subjecting the film to cross-directional strains comprising transverseand longitudinal extensions thereby further orienting said film;coagulating the polymeric material; removing the solvent; and heattreating the resulting film.
 14. The method of claim 13, wherein thepolymeric material in the film is coagulated by quenching in an aqueouscoagulation bath.
 15. The method of claim 14, wherein the solvent isremoved by washing the film with water, and further comprising dryingthe washed film.
 16. The method of claim 13 or 14 wherein the shearstress applied to the film during manufacture is sufficient to producefilm having an ultimate tensile stress in any direction of at least40,000 psi.
 17. The method of claim 16 wherein the shear stress appliedto the film during manufacture is sufficient to produce film having anultimate tensile stress of at least 100,000 psi in at least onedirection.
 18. The method of claim 13 or 14 wherein the shear stressapplied to the film during manufacture is sufficient to produce filmhaving a tensile modulus in any direction of at least 8×10⁵ psi.
 19. Themethod of claim 15 wherein the shear stress applied to the film duringmanufacture is sufficient to produce film having a tensile modulus of atleast 5×10⁶ psi in at least one direction.
 20. The method of claim 13wherein the solvent is polyphosphoric acid, methanesulfonic acid orchlorosulfonic acid.
 21. The method of claim 13 wherein the orderedpolymer a para-ordered, aromatic heterocyclic polymer, an ordered,linear, crystalline polymer containing aliphatic, cycloaliphatic andaraliphatic moieties, a para-ordered aromatic heterocyclic polymerhaving diphenoxybenzene swivel sections in the polymer chain, apara-ordered aromatic heterocyclic polymer containing phenyl groupsalong the polymer chain backbone, or p-phenylenebenzo bisthiazole. 22.The method of claim 13 wherein the ordered polymer is p-phenylenebenzobisthiazole and the solvent is poly phosphoric acid.
 23. The method ofclaim 1, wherein the transverse extension is provided by blowing thefilm and the longitudinal extension is provided by drawing the film. 24.The method of claim 23, wherein the blow ratio is from about 1:1 toabout 3:1.
 25. The method of claim 23, wherein the draw ratio is fromabout 8:1 to about 20:1.
 26. The method of claim 2, wherein thecounter-rotating members are assembled to provide an assembly having aratio of shear zone axial length to annular gap of greater than about50:1.
 27. The method of claim 12, wherein stretching said film comprisesblowing and drawing the film.
 28. The method of claim 27, wherein theblowing and drawing is carried out simultaneously.
 29. The method ofclaim 27, wherein the blow ratio is from about 1:1 to about 3:1.
 30. Themethod of claim 27, wherein the draw ratio is from about 8:1 to about20:1.
 31. The method of claim 12, wherein the counter-rotating dies havea ratio of shear zone axial length to annular gap of greater than about50:1.